Do you wan't your model to rotate in the map? as in a ship turning round on an axis? If so you need to create a func_train and some path points. Hit the rotate box on each path point and then play with the placement of them in the map until you get the movement right.

If you just want to rotate your model on the x axis (I.E. horizontally) then just use the compass controls at the bottom of the entities window.

If you want to rotate on the Y and Z axis then you should probably create a func_train and give it only 1 path point. With then orientate that path point to the rotation you would like using the compass controls.